A view of PKS 2155-304 with XMM-Newton Reflection Grating Spectrometers

We present the high resolution X-ray spectrum of the BL Lac object PKS 2155-304 taken with the RGS units onboard XMM-Newton in November 2000. We detect a OVII Kalpha resonant absorption line from warm/hot local gas at 21.59A (~4.5 sigma detection). The line profile is possibly double peaked. We do not confirm the strong 20.02 A absorption line seen with Chandra and interpreted as z~0.05 OVIII Kalpha. A 3sigma upper limit of 14 mA on the equivalent width is set. We also detect the ~23.5 A interstellar OI 1s-->2p line and derive a factor <=1.5 subsolar O/H ratio in the ISM along PKS 2155-304 line of sight.Comment: 13 pages, 5 figures, 3 tables, emulateapj style. Accepted by Ap

The candidate filament close to the 3C295 galaxy cluster: optical and X-ray spectroscopy

We present a detailed analysis of the overdensity of X-ray sources colse to the 3C 295 galaxy cluster (z=0.46) to assess whether it is associated with a filament of the large-scale structure of the Universe. We obtained optical spectra of the optical counterparts of eleven sources associated with the filament, finding that one is at z=0.474. This is a type 1 AGN at 1.5 arcmin from the cluster center. We found three more sources with a redshift in the range 0.37 - 0.53. We extracted the stacked X-ray spectrum of 47 X-ray sources belonging to the putative filament. We found a significant narrow (at the resolution of the Chandra ACIS-I) line at E ~ 4.4 keV, the energy of the iron Kalpha line at the redshift of the cluster. The detection of this line is confirmed at a confidence level of better than 3sigma and its energy is constrained to be in the range 6.2--6.47 (at a 90% confidence level), excluding an identification with the 6.7 helium-like iron line from the hot cluster ICM at better than 4 sigma. We conclude that the detection of the redshifted line is a strong indication that at least several of the excess sources lie at z ~ 0.46 and that AGNs are efficient tracers of the ``filament'' connected with the central cluster of galaxies.Comment: 7 Pages 9 .ps figures, A&A in pres

The X-ray absorber of PKS2126-158

BeppoSAX observed the z=3.27 quasar PKS2126-158 on 1999 May 24-28 when its 2-10keV and 0.1-2.5keV fluxes were 1.1E-11 and 4.4E-12 cgs respectively, a factor of 2 higher than in all previous ROSAT and ASCA observations and 40% higher than in two more recent Chandra and XMM-Newton observations. The shortest detected rest frame variability timescale is of a few months, comparable to the causal timescale associated to an emission region of ~10 Schwarzschild radii around a few 1E10 solar masses black hole. The source is detected with a signal to noise ratio S/N>=3 up to ~50 keV, 215 keV rest frame. The BeppoSAX observations confirm the presence of low energy absorption along the line of sight, independent on the continuum model adopted, at high confidence level. Despite the limited spectral resolution of the BeppoSAX LECS and MECS it is possible to put constraints on different absorption and continuum models, but not to unambiguously determine the redshift of the absorber. If the absorber is not significantly ionized the BeppoSAX data do prefer an absorber at z<=2.7. Strong and complex metal line systems along the line of sight to PKS2126-158 have been found at z=0.6631 and at 2.64<z<2.82. They could well be associated to the X-ray absorption. Conversely, an ionized (``warm'') absorber at the quasar redshift provides a good fit only if the iron abundance is smaller than ~0.3 solar, while that of the other elements is fixed to the solar value.Comment: A&A in pres

Long term X-ray spectral variability of the nucleus of M81

We have analysed the soft X-ray emission from the nuclear source of the nearby spiral galaxy M81, using the available data collected with ROSAT, ASCA, BeppoSAX and Chandra. The source flux is highly variable, showing (sometimes dramatic: a factor of 4 in 20 days) variability at different timescales, from 2 days to 4 years, and in particular a steady increase of the flux by a factor of >~ 2 over 4 years, broken by rapid flares. After accounting for the extended component resolved by Chandra, the nuclear soft X-ray spectrum (from ROSAT/PSPC, BeppoSAX/LECS and Chandra data) cannot be fitted well with a single absorbed power-law model. Acceptable fits are obtained adding an extra component, either a multi-color black body (MCBB) or an absorption feature. In the MCBB case the inner accretion disk would be far smaller than the Schwartzchild radius for the 3-60X 10^6 solar masses nucleus requiring a strictly edge-on inclination of the disk, even if the nucleus is a rotating Kerr black hole. The temperature is 0.27 keV, larger than expected from the accretion disk of a Schwartzchild black hole, but consistent with that expected from a Kerr black hole. In the power-law + absorption feature model we have either high velocity (0.3 c) infalling C_v clouds or neutral C_i absorption at rest. In both cases the C:O overabundance is a factor of 10.Comment: 30 pages with 15 figures. Accepted for publication in the Astrophysical Journa

FAVOR (FAst Variability Optical Registration) -- A Two-telescope Complex for Detection and Investigation of Short Optical Transients

An astronomical complex intended to detect optical transients (OTs) in a wide field and follow them up with high time resolution investigation is described.Comment: 4 pages, 3 figures. To be published in "Il Nuovo Cimento", Proceedings of the 4th Rome Workshop on Gamma-Ray Bursts in the Afterglow Era, eds. L. Piro, L. Amati, S. Covino, B. Gendr

Probing the complex environments of GRB host galaxies and intervening systems: high resolution spectroscopy of GRB050922C

The aim of this paper is to investigate the environment of gamma ray bursts (GRBs) and the interstellar matter of their host galaxies. We use to this purpose high resolution spectroscopic observations of the afterglow of GRB050922C, obtained with UVES/VLT 3.5 hours after the GRB event. We found that, as for most high resolution spectra of GRBs, the spectrum of the afterglow of GRB050922C is complex. At least seven components contribute to the main absorption system at z=2.1992. The detection of lines of neutral elements like MgI and the detection of fine-structure levels of the ions FeII, SiII and CII allows us to separate components in the GRB ISM along the line of sight. Moreover, in addition to the main system, we have analyzed the five intervening systems between z = 2.077 and z = 1.5664 identified along the GRB line of sight. GRB afterglow spectra are very complex, but full of information. This can be used to disentangle the contribution of the different parts of the GRB host galaxy and to study their properties. Our metallicity estimates agree with the scenario of GRBs exploding in low metallicity galaxiesComment: 11 pages, 12 figures, accepted by A&

The probabilistic random forest applied to the QUBRICS survey: improving the selection of high-redshift quasars with synthetic data

Several recent works have focused on the search for bright, high-z quasars (QSOs) in the South. Among them, the QUasars as BRIght beacons for Cosmology in the Southern hemisphere (QUBRICS) survey has now delivered hundreds of new spectroscopically confirmed QSOs selected by means of machine learning algorithms. Building upon the results obtained by introducing the probabilistic random forest (PRF) for the QUBRICS selection, we explore in this work the feasibility of training the algorithm on synthetic data to improve the completeness in the higher redshift bins. We also compare the performances of the algorithm if colours are used as primary features instead of magnitudes. We generate synthetic data based on a composite QSO spectral energy distribution. We first train the PRF to identify QSOs among stars and galaxies, then separate high-z quasar from low-z contaminants. We apply the algorithm on an updated data set, based on SkyMapper DR3, combined with Gaia eDR3, 2MASS, and WISE magnitudes. We find that employing colours as features slightly improves the results with respect to the algorithm trained on magnitude data. Adding synthetic data to the training set provides significantly better results with respect to the PRF trained only on spectroscopically confirmed QSOs. We estimate, on a testing data set, a completeness of ∼ 86 per cent and a contamination of ∼ 36 per cent. Finally, 206 PRF-selected candidates were observed: 149/206 turned out to be genuine QSOs with z > 2.5, 41 with z < 2.5, 3 galaxies and 13 stars. The result confirms the ability of the PRF to select high-z quasars in large data sets

Scintillation measurements of the millisecond pulsar PSR J0030+0451 and pulsar space velocities

Scintillation observations of the nearby single millisecond pulsar (MSP) PSR J0030+0451 were carried out with the Parkes 64m radiotelescope at three different epochs in 1999. From analysis of the dynamic spectrum we obtained the amplitude of the electron density power spectrum log C_N^2 ~= -3.33$ and a scintillation velocity V_iss <~ 15 km s^-1. This result shows that the Shklovskii effect on the spin-down rate \dot P is negligible. We also performed a correlation analysis between pulsar proper motions (V_pm) and scintillation velocities (V_iss) using updated measurements for a sample of 77 objects, 17 of which are MSPs. The full sample shows a correlation coefficient r_s ~= 80% at an extremely high significance level, while for the MSP sub-sample (excluding 2 outliers) we obtain r_s ~= 90%.Comment: 8 pages, LaTeX, 4 EPS figures, accepted for publication on A&